The present invention relates to an image reading apparatus and method for forming an image of an original on solid-state image sensing devices via an image-forming optical system to read the image.
Various image reading apparatuses that form an image of an original on a plurality of line sensors (solid-state image sensing devices such as CCDs) via an image-forming optical system and that digitally read a black-and-white or colored image based on output signals from the line sensors have been proposed.
FIG. 21 is a schematic view of the integral part of an optical system in a conventional colored-image reading apparatus as seen from a lateral side.
In this figure, 100 is a platen glass on which an original to be read is placed, 101 is a bar-shaped light source for illuminating an original, and 102 is a reflector for improving the illumination efficiency.
An image of an original (not shown) illuminated by the bar-shaped light source 101 and the reflector 102 is guided to an image-forming optical system 104 via mirrors 103-a, 103-b, and 103-c. The image-forming optical system 104 forms the image of the original on solid-state image sensing devices 105.
Together with the light source 101 and the reflector 102, the mirror 103-a moves at a speed (v) in a sub-scanning direction A relative to the original, and in synchronization with the mirror 103-a, the mirrors 103-b and 103-c also move in the sub-scanning direction A at a speed v/2. The line sensor of the solid-state image sensing devices 105 is arranged in a main scanning direction, so a combination of this arrangement with the relative shift of each of the above sections in the sub-scanning direction enables the image of the original to be two-dimensionally scanned and read.
In such a configuration, the image formed on the solid-state image sensing devices 105 is converted into an electric signal and used in various apparatuses. For example, it is sent to an output apparatus (not shown) for printout or to a storage apparatus where it is stored as input image information.
The light source for an image reading apparatus configured in this manner includes a halogen lamp, a fluorescent lamp, and a xenon lamp. Of these lamps, the halogen lamp has been typically used as the light source for such an image reading apparatus. Although the halogen lamp emits light of a high luminance, there are problems in which the temperature of the apparatus significantly increases with the increasing temperature of the lamp and this lamp requires 200- to 300-W power, thereby increasing the power consumption of the entire apparatus.
In order to avoid such problems, the recent trend is to develop fluorescent and xenon lamps of a high luminance as light sources for image reading apparatuses.
Most fluorescent and xenon lamps comprise a bar-shaped hollow tube with a small amount of mercury powders and several Torrs of argon (Ar), krypton (Kr), or xenon (Xe) sealed therein wherein various phosphors are coated on the inner wall of the tube and wherein electrodes are placed at the respective sides of the tube to seal it.
Ultraviolet rays emitted from mercury or various gases due to discharge from the electrodes excite the phosphors coated on the inside of the tube to radiate visible radiation depending on the emission property of the phosphors.
In addition, the phosphors are selected depending on a spectral distribution required for the light source.
In particular, a colored-image reading apparatus requires a light source radiating light of a wide wavelength range including red (R), green (G), and blue (B), and an approach for mixing phosphors of a plurality of colors together and coating the mixture on the inner wall of the tube is used if a light source of a particularly high luminance is required.
In addition, if the quantity of light from a fluorescent or xenon lamp is to be controlled, then instead of controlling the lighting voltage as in the halogen lamp, the pulse width modulation method for controlling the lighting time using a specified current is generally used to control the quantity of light. This is because the fluorescent or xenon lamp is characterized by lighting when current supplied to the lamp exceeds a fixed value and because the method for controlling the quantity of light by varying the magnitude of the current cannot provide a wide control range for the quantity of light.
On the other hand, for image reading apparatuses using a fluorescent or xenon lamp, an approach that omits the above light quantity control and that enables the variable setting of the gain of an amplifier for electrically amplifying output signals from the solid-state image sensing devices so as to correspond to a decrease in the quantity of light over time so that appropriate signal output levels can be obtained by varying the gain according to the decrease in the quantity of light has been proposed. In such a method, however, the gain value may vary the S/N ratio of read signals.
The above conventional examples, however, have the following disadvantages.
In an image reading apparatus using a light source with phosphors acting as a light emitting source as in the fluorescent or xenon lamp, a method of controlling the quantity of light by controlling the duration of a pulse signal corresponding to the lighting time while maintaining a current flowing through the lamp at a specified value.
FIG. 22 shows a waveform of a control signal for controlling the quantity of light from a light source. The horizontal axis in this figure represents time, and the vertical axis represents the value of a current for controlling the quantity of light from the light source and the intensity of light from a fluorescent lamp. In FIG. 22, calibration has been carried out so that the maximum current value and the corresponding intensity of light emitted from the fluorescent lamp are shown at the same point in the vertical-axis direction on the graph.
The Hsync interval on the horizontal axis indicates the time corresponding to one accumulation time period of the solid-state image sensing device, and during this time, charges corresponding to the quantity of light which incidents on a light receiving section of the solid-state image sensing device are accumulated.
For normal pulse width control, a control pulse signal having a predetermined-duration is output once per accumulation time period in synchronization with the leading or trailing edge of a trigger signal indicating the start of the accumulation time period. In this manner, by controlling the quantity of light in synchronization with the trigger signal indicating the start of an accumulation time period, noise in an image signal that results from beat caused by the interference between the accumulation time period and the pulse width control for controlling the quantity of light is conventionally removed.
On the other hand, in a fluorescent or xenon lamp using phosphors as a light source, by coating phosphors of a plurality of colors, it is often used as a white light source having a spectral distribution of a wide wavelength range covering the overall visible radiation range in an image reading apparatus for reading color information.
The use of such a white light source may pose a problem due to the difference in afterglow property among the phosphors. The afterglow property is generally an exponentially decreasing property that is determined by the time during which the phosphors excited by ultraviolet rays remain in a high energy state.
This phenomenon indicates that light emitted from the phosphors may remain despite the instantaneous interruption of a current controlling the emission of light from the light source. Attenuation time T that is the time from the start of attenuation of the intensity of light until it reaches 1/e of the intensity is expressed by the following Equation (1), which depends on the properties of the materials of the phosphor:
T=e(xcfx84xe2x88x921)xe2x80x83xe2x80x83(1)
where xcfx84 is a property determined by the material of the phosphor. If, for example, phosphors corresponding to R, G, and B are mixed as in the white light source used for the above colored-image reading apparatus, the problem caused by the difference in afterglow property occurs due to the difference in xcfx84 value among the R, G, and B phosphors.
In general, the materials of phosphors are determined in terms of its luminescence wavelength property in each wavelength zone, luminescence efficiency, and lifetime expectancy, and the following materials are often used.
Blue: BaMg2Al16027
Central wavelength: 452 nm, T=2 xcexcsec.
Red: Y203: Eu2+
Central wavelength: 611 nm, T=1.1 msec.
Green: LaPO4: Ce, Tb
Central wavelength: 544 nm, T=2.6 msec.
As described above, due to the difference in afterglow property among the colors (in particular, the attenuation time of blue is short), the barycenter of reading positions in the sub-scanning direction may differ among the colors.
This phenomenon will be described with reference to FIG. 22.
Typically, the solid-state image sensing device accumulates charges generated in proportion to the quantity of incident light during the Hsync period corresponding to the one accumulation time period. In addition, the duration in the figure corresponds to the time during which a current for driving the fluorescent lamp is provided in accordance with the duty, and in most cases, the current which oscillates at a high frequency is provided during this duration. After the time corresponding to the duration has passed, the intensity of light attenuates. This attenuation property is determined by the following two factors. One is the attenuation property of a bright-line spectrum emitted by the fluorescent lamp, and the other is the attenuation property of the intensity of light from the phosphors described above. While one accumulation time period corresponding to Hsync is normally several 100 xcexcsec., the attenuation property of the bright-line spectrum is 1 xcexcsec. or less, so it has almost no adverse effects. The attenuation property of the fluorescent lamp, however, ranges up to the ms order, so it significantly affects the total attenuation property. Thus, the attenuation property of the intensity of light is determined by the sum of the above two types of intensities of light and the attenuation property of each phosphor.
The typical afterglow of each of R, G, and B generated depending on the attenuation property is shown FIG. 22. In a fluorescent lamp which has emitted an approximately constant quantity of light over a duration driven by approximately constant amount of current, when the duration is over, the intensity of light corresponding to the bright-line spectrum instantaneously attenuates. The attenuated intensity corresponds to L1 in FIG. 22, and light of the intensity corresponding to L2, i.e. afterglow, attenuates depending on the attenuation property of the phosphors.
The afterglow property of each color conventionally causes the following problems in the image reading apparatus.
First, the one accumulation time period of the solid-state image sensing device is a temporal reference for reading an image as well as a reading position reference for reading the image in the sub-scanning direction. In addition, the pixel density in the main scanning direction with which image information is to be read is determined by the pixel size of the solid-state image sensing device, and the pixel density in the sub-scanning direction is determined by a relative moving distance between an original and a mirror for scanning the original during one accumulation time period during reading the original. Thus, the horizontal axis in the graph of FIG. 22 can be considered as showing the position. The phenomenon in which the barycenter of the quantity of light in the Hsync period is different among colors depending on the afterglow property of each color indicates that the barycenter of the reading position in the sub-scanning direction is offset depending on the color. The offset of the barycenter of the reading position in the sub-scanning direction causes color deviation in the sub-scanning direction, thereby degrading the performance of the image reading apparatus.
The present invention has been made in consideration of the above situation, and an object of the present invention is to prevent color deviation at a reading position in the sub-scanning direction caused by the difference in afterglow property among phosphors of the respective colors in controlling a white light source for irradiating an original with light.
According to the present invention, the foregoing object is attained by providing an image reading apparatus for forming an image of an original on a plurality of line sensors via an image-forming optical system to read the image, comprising: a white light source for irradiating the original with light, the light source having different afterglow properties for different colors corresponding to a plurality of read colors of the plurality of line sensor; control value determining means for determining, based on a quantity of light from the white light source, a control value for the white light source at a predetermined timing so that a barycenter of the quantity of light in the direction of a time axis in one charge accumulation time period almost aligns with the center of the one charge accumulation time period; and light source control means for controlling the white light source based on the control value determined by the control value determining means.
According to the present invention, the foregoing object is also attained by providing an image reading apparatus for forming an image of an original on a plurality of line sensors via an image-forming optical system to read the image, comprising: a white light source for irradiating the original with light, the light source having different afterglow properties for different colors corresponding to a plurality of read colors of the plurality of line sensors; a sensor for measuring a quantity of light from the white light source; control value determining means for determining, based on the statistical amount of the quantity of light from the white light source measured by the sensor, a control value for the white light source so that a barycenter of the quantity of light in the direction of a time axis in one charge accumulation time period almost aligns with the center of the one charge accumulation time period; and light source control means for controlling the white light source based on the control value determined by the control value determining means.
Furthermore, according to the present invention, the foregoing object is also attained by providing the control method, wherein the predetermined number is varied depending on the length of the irradiation time of the white light source in the one charge accumulation time period.
Furthermore, according to the present invention, the foregoing object is also attained by providing a method for controlling an image reading apparatus having a white light source for irradiating an original with light, the light source having different afterglow properties for different colors corresponding to a plurality of read colors of a plurality of line sensors, the image reading apparatus forming an image of the original irradiated in the white light source on the plurality of line sensors via an image-forming optical system to read the image, the method comprising: a measuring step of measuring the quantity of light from the white light source; a control value determining step of determining a control value for the white light source at a predetermined timing on the basis of the statistical amount of the quantity of light from the white light source measured in the measuring step so that the barycenter of the quantity of light in a direction of a time axis in one charge accumulation time period almost aligns with the center of the one charge accumulation time period; and a light source control step of controlling the white light source based on the control value determined in the control value determining step.